Electrophotographic color proofing element and method for using the same

ABSTRACT

An electrophotographic proofing element is described comprising a photoconductive layer on an electrically conducting substrate, capable of transmitting actinic radiation to which the photoconductive layer is responsive, and a dielectric support, releasably adhered to the substrate, comprising the photoconductive layer or an overcoat thereof forming a surface of the element capable of holding an applied electrostatic charge. To use the element, the surface of the dielectric support is charged, and the photoconductive layer is imagewise-exposed to actinic radiation, thereby forming a developable electrostatic image on the dielectric surface. The electrostatic image, in turn, is developed with toner to form a first color image. A composite color image is formed on the element by repeating the sequence one or more times with imagewise exposure of the photoconductive layer to actinic radiation transmitted through the transparent support, and developing over each preceding image with a different color toner. The composite toner image is transferred with the dielectric support to a receiving element to form a color copy such as a three-color filter array or a color proof closely simulating the color print expected from a full press run.

This is a division of application Ser. No. 686,509 filed Dec. 26, 1984,now abandoned.

The present invention relates to an electrophotographic proofing elementadapted to form a multicolor toner image. In particular, the inventionrelates to an electrophotographic element containing a dielectricsupport releasably adhered to a photoconductive element adapted to formtwo or more color toner images.

Color proofing is employed in the color printing field to formrepresentative interim prints from color separation components in theform of screened or unscreened separation negative or positive films ormasks. Through the formation of such proofs, the separation componentscan be evaluated prior to the manufacture of expensive printing plateswith such components to determine whether the separation componentsfaithfully duplicate their contribution to the desired finished product,and whether the color separations formed from the separation componentsare in proper register. If so, the printing plate associated with eachseparation component is prepared and ultimately employed in the pressrun. As is often the case, however, the separation component requiresrepeated alteration to satisfy the user, an expensive proposition if newprinting plates are prepared after each such alteration. Instead,inexpensive proofs serve in this pre-press evaluation of the separationcomponents.

A number of systems are employed in the color-proofing area and aredescribed in detail in U.S. Pat. No. 4,425,417 issued Jan. 10, 1984. Forexample, in one system, presensitized sheets are exposed to lightthrough a separation silver halide negative and developed to provide acolor separation image on a transparent support. The resulting colorseparation images are then overlaid in register on a paper support toprovide a proof.

U.S. Pat. No. 4,358,195 issued Nov. 9, 1982, describes anelectrophotographic proofing element upon which color toner images aresequentially formed and transferred to an element in register withpreviously formed and transferred toner images. The system described inthis patent thus avoids the use of presensitized materials. However,like other systems, the electrophotographic color proofing systemdescribed in U.S. '195 requires that the registration of each colorseparation image on the composite proofing element take place duringeach toner-transfer stage of the process. Unfortunately, registration oftoner separation images at the toner transfer stage of the process is aninherently slow process. Furthermore, proper registration of the imagesis dependent on virtually absolute dimensional stability of the receiverelement to which the images are transferred throughout all the transfersteps. It will be appreciated that it is difficult to prevent stretch orshrinkage or other distortion of the element while it is subjected topressure, heat or liquid toner media during development or transfer.When distortion occurs, image registration suffers. In addition, thetransfer of the toner images from the electrophotographic proofingelement to the receiver as described in U.S. ' 195 can be incomplete,resulting in lower-quality image compared with the quality of an imageachieved with substantially complete transfer of toner.

British Pat. No. 1,035,837, on the other hand, describes anelectrophotographic overprinting method in which color toner separationimages are formed in sequence and in overlapping configuration on aphotoconductive element for simulating, as the patent points out, thehue, intensity, transparency and overprinting characteristics ofstandard lithographic four-color ink systems. To provide foroverprinting of the separation images directly on the photoreceptor, thepatent teaches the use of whitish, light-colored or transparent waxysubstances with its disclosed toners so as to avoid the blanketingeffect of pigments in the toner of previously formed images. It isapparent from this teaching that color quality is sacrificed for thesake of overprinting directly on the photoreceptor. Furthermore, thebackground against which the overprinted toners are viewed does notfaithfully represent the print background against which lithographicinks are viewed after a press run.

In accordance with the invention, an electrophotographic proofingelement is provided which can be employed in a color proofing method asdescribed below to form toner colors truly representative of the pressrun colors desired. When employed in the color proofing method, thiselement provides for viewing of a composite toner image against abackground which is--or closely resembles--the print stock in a pressrun. The proofing element provided by the present invention comprises:

(a) a photoconductive layer on an electrically conductive substratecapable of transmitting actinic radiation to which the photoconductivelayer is responsive and

(b) a releasable dielectric support comprising the photoconductivelayer, or an overcoat thereof, forming a surface of the element capableof holding an applied electrostatic charge.

In the element so defined, the photoconductive layer is charged,imagewise-exposed to actinic radiation, and contacted with anelectrographic developer to form a first toner color separation image onthe dielectric support. The imagewise exposure in forming the firsttoner image can be from the front surface or it can be from the rearsurface, in which case actinic radiation passes through the substrate.However, in the formation of subsequent toner separation imagesoverlapping the first toner image on the dielectric support, imagewiseexposure to actinic radiation is conducted through theradiation-transmissive, electrically conducting substrate. In this way,electrostatic images faithfully corresponding to the exposure patterncan be formed on the dielectric support despite the presence of tonerimages from previous imaging cycles.

Accordingly, in another embodiment of the present invention, amulticolor proofing method is provided employing the above-definedelement. This method comprises:

(1) forming a first color separation image on the dielectric support ofthe electrophotographic proofing element by:

(a) overall charging the surface of the dielectric support,

(b) imagewise-exposing the photoconductive layer to actinic radiation toform a first electrostatic image on the surface of the dielectricsupport, and

(c) developing the first electrostatic image with a first colordeveloper composition to form the first color separation image,

(2) forming a second color separation image over the first colorseparation image on the dielectric support by:

(d) overall charging the surface of the dielectric support and firstcolor separation image,

(e) while the dielectric support and first color separation image arestill charged, imagewise-exposing the photoconductive layer through thesubstrate to form a second electrostatic image on the surface of thedielectric support and first color separation image, and

(f) developing the second electrostatic image with a second colordeveloper composition to form the second color separation image, and

(3) contacting the surface of the dielectric support having the colorseparation images thereon with a receiving element, and

(4) transferring the dielectric support and color separation images tothe receiving element to form a multicolor proof.

The present invention marks the first time that a releasable support isemployed in combination with an actinic radiation-transmissive substratein a color proofing element. The invention provides for convenientregistration of all the separation images directly on the describedelement. In addition, substantially complete transfer of the overlappingcolor separation images is effected as a consequence of simultaneoustransfer with the dielectric support, to produce thereby a high-qualitycolor proof which faithfully resembles the printed product from ananticipated full press run. The transferred dielectric support alsoprovides protection for the toner images sandwiched between thedielectric support and the receiver in the form of abrasion-resistance,and also color stability, particularly if the toner colorants arevulnerable to light or aerial degradation.

DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the drawings in which:

FIG. 1 illustrates an electrophotographic proofing element of theinvention.

FIGS. 2 through 4 illustrate that part of the invention in which a firsttoner separation image is electrophotographically formed on the proofingelement of the invention.

FIGS. 5 through 7 illustrate that part of the invention in which asecond toner separation image is electrophotographically formed over thefirst toner separation image on the proofing element of the invention.

FIG. 8 illustrates a proofing element of the invention upon which fourtoner separation images have been sequentially formed in accordance withthe method of the invention.

FIG. 9 illustrates thermal transfer of the dielectric support and acomposite toner image of four toner color separation images to areceiver to form a color proof in accordance with the final step of theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a unique electrophotographic proofingelement adapted to form a plurality of at least partially overlappingtoner color separation images directly on the proofing element. Itdiffers from electrophotographic annotation in that toner from oneimaging cycle may deposit on toner from prior imaging cycles, whereas inconventional annotation toner images are sequentially added usually onlyto untoned regions of the photoreceptor. To form such overlapping tonerseparation images directly on the electrophotographic element, it isnecessary to provide means by which the photoconductive portion of theelement can be imagewise-illuminated in such a manner as to form latentelectrostatic images on untoned, as well as toned, regions of thephotoreceptor. Furthermore, after each toner separation image is formedin overlapping configuration, means must be provided to view thecomposite image resulting from such overlapping images on the printstock (usually paper) upon which the multicolor press run prints will bemade after approval of the color proof. The present invention providesfor these objectives and other advantages through the use of anelectrically conducting substrate capable of transmitting actinicradiation to which the photoconductive layer is responsive, and throughthe use of a releasable dielectric support forming a charge-holdingsurface of the element.

The electrophotographic proofing element of the present inventioncomprises a photoconductive layer on an electrically conductingsubstrate transmitting actinic radiation for the photoconductive layer,and a dielectric support releasably adhered to and forming a chargeablesurface of the element. The dielectric support, as previously noted,comprises the photoconductive layer, in which case the element cancomprise the photoconductive layer--functioning also as the dielectricsupport--releasably adhered to the electrically conducting substrate.Alternatively, the releasable dielectric support can comprise anovercoat on the photoconductive layer. The latter is preferred, and thediscussion below is directed predominantly to the use of a suchembodiment for purposes of illustrating the invention.

Any type of material can be employed as the photoconductive layer whichis capable of charge carrier formation under the influence ofelectrostatic charging and exposure to actinic radiation. Inasmuch asthe present element can be discarded after the formation of the proof,single-use photoconductive layers are preferably employed.Representative materials and layers include polyarylamines or arylalkanephotoconductors as described, for example, in U.S. Pat. No. 4,442,193,the disclosure of which is incorporated herein by reference. Thephotoconductors described in U.S. '193 include:

(1) arylamines, diarylamines, nonpolymeric and polymeric triarylaminessuch as tri-p-tolylamine;

(2) polyarylalkanes such as triphenyl methanes and tetraphenyl methanes,for example, bis(4-diethylamino)tetraphenyl methane;

(3) 4-diarylamino-substituted chalcones;

(4) nonionic cycloheptenyl compounds;

(5) compounds containing an:

    ═N--N═                                             I

nucleus;

(6) organic compounds having a 3,3'-bis-aryl-2-pyrazoline nucleus;

(7) triarylamines in which at least one of the aryl radicals issubstituted by either a vinyl radical or a vinylene radical having atleast one active hydrogen-containing group;

(8) triarylamines in which at least one of the aryl radicals issubstituted by an active hydrogen-containing group;

(9) any other organic compound which exhibits photoconductive propertiessuch as those set forth in U.S. Pat. No. 3,250,615 and Australian Pat.No. 248,402, and the various polymeric photoconductors such as thephotoconductive carbazole polymers described in U.S. Pat. No. 3,421,891.

In preferred photoconductive compositions, arylalkane leuco basephotoconductors are the principal photoconductive constituents. Suchpreferred compositions are advantageously nonpersistently conductive andsensitive to radiation below 400 nm, but substantially insensitive toradiation above 400 nm. Arylalkane leuco base photoconductors aredisclosed, for example, in U.S. Pat. No. 3,542,547 above and bear thestructure: ##STR1## wherein:

each of R and R' is selected from the group consisting of hydrogen,alkyl and aralkyl having 1-4 carbon atoms in the alkyl group;

each of X' and X" is selected from the group consisting of alkyl having1 to 4 carbon atoms, alkoxy having 1-4 carbon atoms, hydroxyl andhalogen;

each of Y' and Y" is selected from the group consisting of alkyl having1-4 carbon atoms, alkoxy having 1-4 carbon atoms, hydroxyl, halogen andhydrogen; and

each of A and B is:

(1) hydrogen, with the proviso that A and B are not both hydrogen;

(2) aryl such as phenyl, α-naphthyl, β-naphthyl, 9-anthryl andsubstituted derivatives thereof wherein the substituent is dialkylamino,alkylamino, amino, alkyl, alkoxy, hydroxyl or halogen;

(3) an aliphatic alkyl group having 1-18 carbon atoms, e.g., methyl,ethyl, propyl, butyl, isobutyl, octyl, dodecyl, etc., including asubstituted alkyl group having 1-18 carbon atoms;

(4) a cycloalkyl group having 4-8 carbon atoms in the cyclic nucleus,e.g., cyclobutyl, cyclohexyl, cyclopentyl, etc., including a substitutedcycloalkyl group; or

(5) a cycloalkenyl group having 4-8 carbon atoms in the cyclic nucleus,e.g., cyclohex-3-enyl, cyclopent-3-enyl, cyclobut-2-enyl,cyclohex-2-enyl, etc., including a substituted cycloalkenyl group.

Representative Formula II arylmethane photoconductors are set forth inTable 2 of U.S. Pat. No. 3,542,547. Particularly useful photoconductivecompositions comprise crystallization-inhibiting mixtures of two or moreof the arylalkane leuco base photoconductors as disclosed in U.S. Pat.No. 4,301,226 issued Nov. 17, 1981, to L. E. Contois et al. A preferredcrystallization-inhibiting mixture comprises three arylmethanephotoconductors:bis(4-N,N-diethylamino-2-methylphenyl)-4-methylphenylmethane,1,1-bis(4-N,N-diethylamino-2-methylphenyl)-2-methylpropane and4,4'-bis(diethylamino)-2,2'-dimethyltriphenylmethane.

The total amount of photoconductor in the defined composition may varywidely, but preferably ranges from about 5 to about 40 weight percentbased on the solvent-free weight of the layer.

Photoconductors described above can be incorporated into an electricallyinsulating binder and coated as photoconductive layers on a transparentelectrically conductive support to form all or a portion of theelectrophotographic proofing element of the invention, depending onwhether the photoconductive layer is also the releasable dielectricsupport.

Preferred electrically insulating binders for use in preparing thephotoconductive layers are film-forming, hydrophobic polymeric bindershaving fairly high dielectric strength. Materials of this type comprisestyrene-butadiene copolymers; silicone resins; styrene-alkyd resins;silicone-alkyd resins; soya-alkyd resins; poly(vinyl chloride);poly(vinylidene chloride); vinylidene chloride-acrylonitrile copolymers;poly(vinyl acetate); vinyl acetate-vinyl chloride copolymers; poly(vinylacetals) such as poly(vinyl butyral); polyacrylic and polymethacrylicesters such as poly(methyl methacrylate), poly(n-butyl methacrylate),poly(isobutyl methacrylate); polystyrene; nitrated polystyrene;polymethylstyrene; isobutylene polymers; polyesters such aspoly[ethylene-co-alkylenebis(alkyleneoxyaryl)phenylenedicarboxylate];phenolformaldehyde resins; ketone resins; polyamides; polycarbonates;polythiocarbonates;poly(isopropylidene-bis-phenoxyethylene-co-terephthalate]);poly[ethylene-co-isopropylidene-2,2-bis(ethyleneoxyphenylene)terephthalate]; copolymers of vinylhaloarylates; poly(ethylene-co-neopentyl terephthalate); and vinylacetate such as poly(vinyl-m-bromobenzoate-co-vinyl acetate).

Especially useful binders are polyesters having relatively high glasstransition temperature (Tg) values and being able to accomodate highphotoconductor concentrations without the photoconductors migrating fromthe photoconductive layer toward the dielectric support. If suchmigration occurs, photoconductive compounds, often undesirably colored,can ultimately pass to the receiving element when the support istransferred in the color proofing method described herein. Use ofpolymers with a relatively high Tg, moreover, prevents unwanted chargeinjection when the photoconductive layer is also the releasabledielectric support as discussed in greater detail below. Examples ofsuch relatively high Tg polyesters arepoly[(4,4'-norbornylidene)bisphenoxyethylene-co-ethylene terephthalate];poly[(4,4'-hexahydro-4,7-methanoindene-5-ylidene)bisphenoxyethylene-co-ethyleneterephthalate]; poly[(isopropylidene)bisphenoxyethylenecoethylenenaphthoate]; poly(1,2-propylene naphthoate);poly[(isopropylidene)bisphenoxyethylene-co-1,2-propylene naphthoate];poly[(4,4'-isopropylidene)-bisphenoxyethylene-co-bis(1,2-propylene)naphthoate]; andpoly[1,2-propylene methylsuccinate-co-naphthoate].

The photoconductive layers employed in the above-defined element arepreferably selected so as to provide sensitivity below 400 nm only andexhibit nonpersistent conductivity. (400 nm represents the approximatespectral wavelength of transition between visible and nonvisible light;persistent conductivity refers to the lingering conductivity of somematerials in exposed regions.) Such sensitivity below 400 nm permitscharging, exposure and development of the photoconductive composition invisible light.

Nonpersistent conductivity is important because the photoconductivelayer is to be cycled through a charge-expose-develop sequence a numberof times to produce a multicolor proof. If the photoconductive layer ispersistent, that is, if the layer remains conductive in light-struckregions from preceding exposure steps, unwanted image formation fromsuch preceding steps will contaminate subsequent images desired.Specifically, it is desirable that the period during which thephotoconductive layer remains conductive after exposure to actinicradiation be less than 30 seconds.

To further enhance the photosensitivity of the photoconductive layer,sensitizers are preferably incorporated therein. These sensitizers arepreferably sensitizers for the region of the electromagnetic spectrumbelow 400 nm exclusively to further provide roomlight handleability ofthe element during use. Particularly useful sensitizers providing suchsensitivity are described in the above-mentioned U.S. Pat. No.4,442,193. These sensitizers are 1,4,5,8-naphthalene dicarboximidecompounds and are particularly useful as sensitizers for arylalkaneleuco base photoconductors. The preferred structure of these compoundsis: ##STR2## wherein:

R¹ and R², which may be the same or different, represent aryl, such asphenyl or naphthyl, or aryl substituted with alkyl, alkoxy,perfluoroalkyl or perfluoroalkoxy groups having 2-20 alkyl carbon atoms;sulfonyl; sulfone; sulfonamide; nitrile; or nitro groups;

R³, R⁴, R⁵ and R⁶, which may be the same or different, representhydrogen, alkyl having 1-4 carbon atoms, alkoxy having 1 to 4 carbonatoms, or halogen; and

n is 0 to 3.

Preferably, R¹ and R² in Formula I are phenyl or phenyl substituted withalkyl, alkoxy or perfluoralkyl, and R³, R⁴, R⁵ and R⁶ are hydrogen.

Representative 1,4,5,8-naphthalene bis-dicarboximides employed in thephotoconductive layer includeN,N'-bis[p-(n-butyl)phenyl]-1,4,5,8-naphthalene bis-dicarboximide;N,N'-bis(m-trifluoromethylphenyl)-1,4,5,8-naphthalene bis-dicarboximide;N,N'-bis(3-phenylpropyl)-1,4,5,8-naphthalene bis-dicarboximide;N,N'-bis[p-(n-actyloxy)phenyl]-1,4,5,8-naphthalene bis-dicarboximide;N,N'-bis(phenyl)-1,4,5,8-naphthalene bisdicarboximide.

The amount of dicarboximide employed can vary widely in accordance withthe degree of sensitization desired. Effective amounts of the sensitizerrepresented by Structure III can vary widely to provide sufficient speedto the photoconductive layer. The optimum concentration in any givencase will vary with the specific photoconductor and sensitizing compoundused. Substantial speeds can be obtained where a sensitizer according toStructure III is added in a concentration range from about 0.0001 toabout 30 percent based on the weight of the composition on a dry basis.A preferred sensitizer concentration range is from about 0.005 to about5.0 percent.

The thickness of the photoconductive layer can vary depending on thetype of photoconductors employed in the layer, speeds desired and otherfactors. Photoconductive layer thicknesses in the range from about 2 toabout 20 micrometers are useful, while layer thicknesses of from about 5to about 13 micrometers are preferred.

As can be seen from the foregoing discussion, the photoconductive layeris often colored as a result of the photoconductive, as well assensitizing, constituents in the layer. For this reason, it is preferredto use a separate overcoat on the photoconductive layer as thereleasable dielectric support; otherwise, after transfer of thecomposite toner image to a receiver as discussed below, the resultingcolor proof may contain undesirable hue as a result of such consituentsin the photoconductive layer if the latter is employed as the releasabledielectric support. It will be appreciated, however, that transparentand colorless photoconductive layers can be prepared for use in theelement, in which case such minor difficulties would be overcome.Furthermore, the presence of any hue from the photoconductive layer maynot be objectionable in some user applications.

In a preferred embodiment, a separate dielectric support, is releasablyadhered to the photoconductive layer. To facilitate such releasability,the photoconductive layer can contain release addenda. Useful andpreferred release addenda are block copolyesters of polysiloxanescorresponding to the structure: ##STR3## wherein:

R is an alkylene radical containing at least 3 carbon atoms;

R¹ is an alkyl radical and R² is an alkyl, alkaryl, aralkyl or arylradical;

A is an alkylene or arylene radical;

a is an integer of at least 10;

b is an integer of at least 1;

c is an integer of at least 2; and

d is an integer of at least 2;

as described in U.S. Pat. No. 3,861,915, the disclosure of which isincorporated herewith by reference. The release agents in U.S. '915 canbe prepared by reacting dihalopolydiorganosiloxane with an aromatic oraliphatic diol to form the dihydroxypolydiorganosiloxane subsequentlyreacted with an aliphatic dicarboxylic acid halide to form the blockcopolyester shown.

Alternative release agents which can also be used are: ##STR4## wherein:

a, b, c, d, R, R¹, R² and A are the same as described above and

R¹¹ is an alkyl group of at least 2 carbon atoms synthesized by reactinga diol-containing polydiorganosiloxane and an aliphatic or aromatic diolwith an aromatic or aliphatic dicarboxylic acid chloride to form thealternative block copolyester indicated. A second alternative blockcopolyester release agent corresponding to the structure: ##STR5##wherein:

R, A, R¹¹, b and d are the same as above and

R¹¹¹ is an alkyl group,

can be employed. The second alternative is formed by reacting amonohalopolydiorganosiloxane polymer with an aliphatic or aromatic dioland an aromatic or alkyl dicarboxylic acid halide.

Representative block copolyesters which can be employed as releaseagents includepoly[4,4'-isopropylidenediphenylene-co-block-poly-(dimethylsiloxanediyl)sebacate];poly[1,4-butylene-co-block-poly(dimethylsiloxanediyl)sebacate];poly[4,4'-isopropylidenedipheylene-co-block-poly(dimethylsiloxanediyl)glutarate;poly(4,4'-isopropylidenediphenylene terephthalate-co-azelate50/50-bdimethylsiloxane); poly(4,4'-isopropylidene-diphenyleneadipate-b-dimethlysiloxane);poly(4,4'-[hexahydro-4,7-methanoinden-5-ylidene]bis(phenylene)carbonate-b-dimethylsiloxane)and poly[1,4-butylene-co-block-poly(dimethylsiloxanediyl)adipate].

The concentration of the release agent can vary to facilitate release ofthe dielectric support as desired. A useful concentration of releaseagent is from about 0.05 percent to about 2 percent by weight of thephotoconductive layer.

The electrically conducting substrate employed in the above-definedelement can comprise any material which is electrically conducting andtransmits actinic radiation to which the photoconductive layer isresponsive. As noted previously, preferred elements of the invention aresensitive to ultraviolet radiation below 400 nanometers, in which casethe electrically conducting substrate should transmit radiation below400 nanometers. In this manner, the photoconductive layer can beimagewise-exposed to actinic radiation through such substrate to formelectrostatic field image patterns on the dielectric support even thoughthe latter contains toner images from previous cycles.

The conducting substrate, furthermore, can either be a single layer ofappropriate electrical conductivity and transmissive capability, or itcan be multilayer comprising, for example, an electrically conductinglayer on a polymeric film. Multilayer substrates are preferred. In suchpreferred substrates, the polymeric film can comprise any suitablepolymer such as cellulose acetate, poly(ethylene terephthalate),polyethylene or polypropylene. The conducting layer on the film,furthermore, can be any well-known transmissive conducting material suchas one formed by coating onto the film a solution of a conductive orsemiconductive material and a resinous binder in a volatile solvent andevaporating the solvent to leave a conductive layer. Particularly goodconductive layers for use with the present elements utilize ametal-containing semiconductor compound such as cuprous iodide, silveriodide, and others as disclosed in U.S. Pat. No. 3,428,451, or metallicsalts of a carboxyester lactone of a maleic anhydridevinyl acetatecopolymer and others dispersed in a binder. Representative bindersinclude poly(vinylformal), poly(vinyl alcohol), poly(vinyl acetate) andmixtures thereof.

The conductive layer can also contain addenda to improve the uniformityof charge on the proofing element during use. In some instances, whenelements are overall charged prior to exposure, the charge so applied isnot entirely uniform but instead is mottled. To avoid or minimize suchmottle, the conductive layer can also contain certain addenda such asfluorocarbon surfactants. Other addenda which can be incorporated intothe conductive layer to minimize mottle include silicones andpolyalkylene oxides such as copolymers of polyethylene oxide andpolypropylene oxide. The concentration of the mottle-reducing addendacan be in the range from about 0.0002 to about 20 percent, by weight,based on the total weight of the conductive layer.

The conductivity of the defined substrate is generally determined interms of surface resistivity. The surface resistivity of usefulsubstrates is less than 10¹¹ ohms per square and is preferably less than10⁵ ohms per square.

As is well-known to those skilled in the art, many arrangements ofphotoconductive layers on electrically conducting substrates result inelements bimodal in nature; i.e., they are capable of producingdevelopable electrostatic images when charged either negatively orpositively and exposed to actinic radiation. However, in some of theseelements, the conducting substrate has a tendency to indiscriminantlyinject either positive or negative charge carriers into thephotoconductive layer before exposure, preventing operation in eitherthe negative or positive charging mode. In view of such behavior, thepresent element can also contain a barrier layer between theelectrically conducting substrate and the photoconductive layer toprevent unwanted charge carriers from entering the photoconductivelayer. Representative barrier layers include cellulose nitrate layers asdisclosed in U.S. Pat. No. 3,640,708 and others as disclosed in column 6of U.S. Pat. No. 3,428,451.

In accordance with a preferred embodiment of the invention, a dielectricsupport in the form of an overcoat is releasably adhered to thephotoconductive layer to form a chargeable surface of the proofingelement. Among other things, the toner separation images are overlappedin sequence on the dielectric support and the toner images, togetherwith the support, are transferred from the photoconductive layer to asuitable receiver to form the desired color proof. Because the receiveris usually representative of the print stock upon which the press runwill take place, the dielectric support is preferably opticallytransparent to facilitate viewing of the toner composite image againstthe receiver, thereby simulating a press run print.

Preferred dielectric supports can be any material capable of retainingan electrostatic charge applied to its surface. In general, anychargeable polymeric material can be employed. Preferably the support issuch that with toner formed on its surface as described herein for thecolor proofing process, forms a barrier to (i.e., prevents) darkinjection of charge from such toner into the photoconductive layer. Thesupport preferably also prevents the transmission to its surface ofcharge carriers photogenerated within the photoconductive layer. If thereleasable dielectric support comprises the photoconductive layer,charge injection from the surface thereof to within the layer or fromwithin the layer to the surface can be minimized or avoided through theuse of polymers with high glass transition temperatures (Tg) as notedpreviously in the discussion relative to polymeric binders for use inthe photoconductive layer. In this regard, polymers with Tg's from about100° C. and higher have been found useful to prevent unwanted chargeinjection. Alternatively, polymers with Tg's below 100° C. can beemployed if the Tg of the photoconductor employed is such that the Tg ofthe polymer and photoconductor in combination is greater than 100° C.When the releasable dielectric support displays such barrier properties,the quality of sequential overlapping separation images is greatlyimproved.

Representative polymers which can be employed in a preferred dielectricsupport include polyesters, vinyl polymers, acrylic polymers orcellulosic polymers. For example, such materials as polyvinyl acetate,cellulose acetate butyrate and copolymers of vinylacetate and crotonicacid can be employed. Mixtures of cellulose acetate butyrate with eitherpolyvinyl acetate or with copolymers of vinylacetate and crotonic acidcan also be employed.

The thickness of preferred dielectric supports can vary to provide thenecessary electrical and optical characteristics to the support. Suchdielectric support can have a thickness in the range from about 0.5 toabout 5.0 micrometers, preferably a thickness from about 1.5 to about3.0 micrometers.

Preferred dielectric supports can be applied to the photoconductivelayer by any of a variety of ways employed in the art to form overcoatedlayers. Preferably, the support is applied from a coating solution ofthe polymeric binder for the support and a coating solvent. To providecoating and layer uniformity, the coating solution can also contain asurfactant such as a silicone fluid, for example, a methylphenylsilicone fluid.

Further advantages can also be obtained when preferred dielectricsupports contain an agent to improve ambient stability. Usefulstabilizing agents include condensates of ethylene oxide withhydrophobic bases formed by condensing propylene oxide with propyleneglycol (polyethylenepolypropylene oxides). These condensates areavailable commercially as Pluronics from Wyandotte Chemicals Corp., acorporation of Wyandotte, Mich. Employing such condensates appears toprovide a degree of hygroscopicity and thereby aids in the electrostaticimage-forming process at lower levels of relative humidity. Theconcentration of the stabilizing agent is from about 0.1 to about 5percent by weight, based on the total weight of the dielectric support.

FIG. 1 depicts a proofing element 1 in accordance with a preferredembodiment of the invention. Element 1 comprises a dielectric support 2releasably adhered to a photoconductive layer 3 to form a chargeablesurface of element 1. Photoconductive layer 3, in turn, overlies atransparent, electrically conductive substrate 4 which in the embodimentshown comprises an electrically conducting layer 5 on a film support 6.Optionally (not shown), element 1 can contain a barrier layer betweenthe photoconductive layer 3 and the electrically conductive substrate 4to facilitate bimodality as described elsewhere herein.

FIGS. 2 through 4 show the formation of the first toner separation imageon the surface of the dielectric support. Initially (FIG. 2), thedielectric support 2 is overall-charged with an appropriate chargingmeans such as a corona charger (not shown) to form a uniform potentialon the surface of the dielectric support 2. Upon being so charged, inthis case positively, a balancing negative charge forms uniformly in theconductive layer 5 as shown. When the photoconductive layer 3 isimagewise-exposed through the conductive substrate 4 to actinicradiation 8 transmitted by overall exposure of a color separationtransparency 9 as shown in FIG. 3, mobile charge carriers, in this casepositively charged holes, are formed in photoconductive layer 3 andmigrate toward the interface of photoconductive layer 3 and conductinglayer 5. (Nonmobile charge carriers, electrons, remain randomlydistributed throughout the photoconductive layer.) Accordingly, theelectric-field strength in exposed regions is diminished while the fieldstrength in unexposed regions remains approximately the same. As aresult, an electrostatic differential pattern is formed on thedielectric support 2 corresponding to the pattern on the transparency.When the electrostatic differential pattern is contacted with apositively charged electrographic developer composition, positivelycharged toner particles adhere to exposed regions exhibiting higherfield strengths and thus form the first toner separation image 10 asshown in FIG. 4. A development electrode (not shown) can also beemployed in the development zone above dielectric support 2. Thedevelopment electrode is usually charged to the same polarity andpotential as the unexposed regions of the dielectric support to enhancethe development efficiency in large solid areas in accordance with knownprocedures. In these illustrations, the first toner separation image 10is a negative sense image in which image optical density correspondsinversely to optical density in the separation transparency. (It will beappreciated by those skilled in the art, however, that toner particlescan be directed to unexposed regions where the surface potential isgreater by using, in the case illustrated, negatively chargeddevelopers.) After developing the first charge pattern, the tonerseparation image 10 can be fixed by pressure, by heat, by solventtreatment or through the use of self-fixing developer formulations asdisclosed, for example, in copending U.S. Pat. application serial No.390,487 entitled SELF-FIXING LIQUID ELECTROGRAPHIC DEVELOPERS CONTAININGPOLYESTER TONERS AND DISPERSED WAX AND PROCESSES FOR USING THE SAME, andfiled June 21, 1982, in the name of D. Santilli.

FIGS. 5 through 7 depict the formation of the second toner separationimage on the dielectric support overlapping the first toner image.Although the figures do not show it, any residual charge remaining onelement 1 after forming the first toner image can be erased by overallexposure to actinic radiation. Thereafter, element 1 is again chargedpositively as shown in FIG. 5 to form a uniform potential on both tonedand untoned images. Imagewise exposure of the photoconductive layer 3through conductive substrate 4 as shown in FIG. 6 to actinic radiation11 transmitted by a second color separation transparency 12 (which isdifferent from color separation transparency 9 shown in FIG. 3) producesa second differential charge pattern in the same manner as discussedabove. Development with a second developer composition, also chargedpositively, results in a second toner separation image 13 partiallyoverlapping the first toner separation image 10 as shown in FIG. 7.

In the formation of a multicolor proof, the above charge-expose-developsteps can be repeated at least two more times to form a four-color imageas shown in FIG. 8 wherein third (14) and fourth (15) toner separationimages overlap first (10) and second (13) toner separation images. Whenthe color of these toners is respectively black, cyan, magenta andyellow for each separation image, a full color composite image resultson dielectric support 2 corresponding to the composite image desired ina press run.

After formation of the desired number of separation images, thedielectric support and the images thereon are brought into contact witha receiver element and transferred from the proofing element to thereceiver to form the desired multicolor proof. FIG. 9 depicts suchtransfer wherein dielectric support 2 on element 1 is brought intocontact with a receiver element 16 in a nip formed between rollers 17and 18. By the application of sufficient pressure and heat within thenip, dielectric support 2 and the composite of images 10, 13, 14 and 15become laminated to the receiver 16 as shown to form a color proof 19.Alternatively (not shown), the dielectric support carrying the tonerimages can be brought into planar contact--as opposed to rollingcontact--with the receiver element where lamination is effected again byheat and pressure provided, for example, by a platen supporting eitherthe receiver or the proofing element. Thereafter, the photoconductivelayer 3 can be stripped from the dielectric support to complete thetransfer of the support and composite image to the receiver, formingthereby the desired color proof comprising, in sequence, the receiver,the composite image and the dielectric support.

In the lamination and subsequent transfer of the dielectric support andcomposite image, a temperature above the Tg of the polymeric material inthe dielectric support is chosen so as to soften and tackify thesupport. The dielectric support, moreover, is chosen from amongmaterials adhering preferentially to the receiver, thereby facilitatingtransfer of the support and composite image to the receiver.Representative temperatures employed during the transfer step extendfrom about 100° to about 115° C.; however, the system can operate over awider temperature range depending on the choice of materials. Practicalconsiderations in selecting the upper temperature point include theliklihood that either the receiver or the proofing element will distortat the temperature selected, as well as the propensity of components inthe photoconductive layer to oxidize at the temperature chosen whichwould tend to impart undesirable background coloration to the finalproof if any of these components transfer to the receiver. The lowerlimit of the temperature employed to effectuate transfer, moreover, isgoverned to a large extent by the permanency of adhesion of the supportand composite image to the receiver. If the support and image are easilyremoved from the receiver, the transfer temperature can be increased toenhance the permanency of such adhesion.

Pressures employed to transfer the support and composite image to thereceiver can also vary widely. Planar transfers (i.e., transferseffected while the dielectric support and receiver are in full planarcontact) have been successful using relatively low pressures from about30 to about 40 pounds per square inch (from about 0.207 to about 0.276megapascals, MPa). Optimum transfers have been effected with a pressureof about 35 psi (about 0.276 MPa); however, departures from suchpressures are appropriate depending, for example, on the roughness ofthe receiver surface. Preferably, the receiver surface should be smoothas rough surfaces tend to inhibit transfer of the support and compositeimage.

The receiver element to which the dielectric support and composite tonerimage are transferred can be any suitable material against or throughwhich the toner image can be viewed. As noted earlier, the receiver ispreferably print stock such as paper upon which the press run will beconducted. The receiver can also be of transparent material such as apolymeric film. With respect to the latter, the present invention alsocontemplates, as an embodiment, transfer of the composite toner imageand dielectric support to image-bearing elements such as microfilm ormicrofiche so that the composite color image forms information inaddition to image information already present on such image-bearingelements. In addition, the invention contemplates the use ofnonbirefringent translucent polymeric materials such as cellulose estersfor use as the receiver. Receivers manufactured from such materials aresuited for use in forming three-color filter arrays by the processdescribed herein involving the formation of filter array matrices of thecomplementary colorants cyan, magenta and yellow in the respective colortoner separation steps. If desirable, the receiver can also contain asuitable overcoat layer adapted to soften under the influence ofpressure and heat during the transfer step. In this manner, the adhesionof the dielectric support and composite toner image to the receiver canbe enhanced.

The following preparations and examples are provided to aid in thepractice and understanding of the invention

EXAMPLE 1 Electrophotographic proofing elements of the invention

An electrophotographic element comprising, in sequence, aphotoconductive layer, a barrier layer, an electrically conducting layerand a film support were employed. A dielectric support was releasablyadhered to the photoconductive layer. The photoconductive layercontained:

(a) as an electrically insulating binder, a blend of two polyesterscontaining 94%, based on the total weight of binder, of a terpolymer ofterephthalic acid, 2,2-bis[4(β-hydroxyethoxy)phenyl]propane and ethyleneglycol; and about 6%, based on the total weight of binder, of acopolymer of terephthalic acid, ethylene glycol and neopentyl glycol;

(b) 22.5%, based on the total weight of the layer, of equal parts byweight of three photoconductive compounds. The three compounds were4,4'-bis(diethylamino)-2,2'-dimethyltriphenylmethane,1,1-bis(4-N,N-diethylamino-2-methylphenyl)-2-methylpropane, andbis(4-N,N-diethylamino-2-methylphenyl)-4-methylphenylmethane;

(c) 2%, by weight of the layer, of the sensitizerN,N-bis[p-(n-butyl)phenyl]-1,4,5,8-naphthalenebisdicarboximide; and

(d) 0.25%, by weight of the layer, of the siloxane-release agentpoly(Bisphenol A)adipate-poly(dimethylsiloxane) to facilitate thermalrelease of the dielectric support.

The barrier layer was optically transparent and comprised cellulosenitrate.

The electrically conducting layer was optically transparent andcomprised (a) cuprous iodide in a binder containing poly(vinyl formal)with 5-7 weight percent, based on binder, poly(vinyl alcohol) and 40-50weight percent poly(vinyl acetate) to provide a surface resistivity of1×10⁴ ohms/square, and (b) 0.26 weight percent, based on the weight ofthe layer, of Fluorad FC-431™ surfactant (a 3M Company tradename for afluorocarbon surfactant described as a short-chain oligomeric esterwherein the ester group is fluorinated).

The support comprised an optically transparent, 4.7 mil (0.12 cm) thickpolyethylene terephthalate film subbed to provide adhesion with theelectrically conducting layer.

The dielectric support on the photoconductive layer comprised a blend ofpoly(vinylacetate-co-crotonic acid, 95/5 mole ratio) and celluloseacetate butyrate in a ratio by weight of 80:20. Several elements wereformed with dielectric supports having uniform thicknesses of 0.5, 1.6or 2.4 micrometers on each respective element.

EXAMPLE 2 Multicolor proofing method

The element described in Example 1 having a 2.4-micrometer thickdielectric support was employed in a color proofing process. A colorseparation negative was mounted on a vacuum platen composed of quartzglass for transparency. The color proofing element was placed inregister over the separation negative with the dielectric supportsurface facing away and the transparent polymeric film facing againstthe negative. To form the first toner separation image on the dielectricsupport, the latter was overall-charged to +600 volts. Thereafter, thephotoconductive layer was imagewise-exposed through the platen,separation negative, transparent film support and conducting layer toform a latent electrostatic image on the dielectric support. The latentimage was developed with a self-fixing liquid electrographic developercontaining black-pigmented toner particles in an electrically insulatingcarrier liquid, followed by a vacuum drying step and a rinse in clearcarrier liquid. The developed image on the dielectric support was thenfused by drying in air. The procedure was repeated three times withcyan-, magenta- and yellow-colored self-fixing developers, respectively,to form a composite multicolor toner image on the dielectric support.

The proofing element bearing the multicolor toner image was then movedto a separate lamination device comprising heated metal and rubberrolls, together forming a nip. The toner image was passed through thenip with and against a receiver paper at a roll temperature of 100° C.(212° )and a pressure of 225 pounds per square inch (1.551 MPa) toeffect transfer of the dielectric support and composite image to thereceiver. The resulting multicolor proof presented a multicolor tonerimage against a paper white background, thus accurately resembling amulticolor print from a full press run.

The black toner employed in the developer composition comprised, asbinder, the polyesterpoly[neopentyl-4-methylcyclohexene-1,2-dicarboxylatecoterephthalate-co-5-(N-potassio-p-toluenesulfonamidosulfonyl)isophthalate)];as colorant, carbon black and alkali blue dye; wax and dispersing agentfor the wax, with the ratio of the weight of the wax plus dispersingagent for wax to the weight of binder being at least 0.25. In the cyan,magenta and yellow toners, the black colorants were replaced withSunfast Blue NF (Sun Chemical Co.), Bonadur Red (Sun Chemical Co.) andRagoon Yellow (Sun Chemical Co.), respectively.

EXAMPLE 3 Proofing element with releasable photoconductive layer

An element as described in Example 1 is prepared omitting the overcoatas the releasable dielectric support. Suitable addenda are incorporatedinto the photoconductive layer to promote its releasability from theelectrically conducting substrate. Preferably, the binder polymeremployed in the photoconductive layer is selected from among relativelyhigh Tg materials to provide a barrier to dark charge injection andunwanted injection of photogenerated charge carriers.

Similar results can be expected when this element is employed in themethod described in Example 2.

Although the invention has been described in considerable detail withparticular reference to certain preferred embodiments thereof, variationand modifications can be effected within the spirit and scope of theinvention.

What is claimed is:
 1. A color proofing method comprising:(I) forming afirst color separation image on the dielectric support of anelectrophotographic proofing element comprising,(a) a photoconductivelayer on an electrically conductive substrate capable of transmittingactinic radiation to which the photoconductive layer is responsive, and(b) a releasable dielectric support comprising the photoconductive layeror an overcoat thereof, forming a surface of the element capable ofholding an applied electrostatic charge, by(i) overall charging thesurface of the dielectric support, (ii) imagewise-exposing thephotoconductive layer to actinic radiation to form a first electrostaticimage on the surface of the dielectric support, (iii) developing thefirst electrostatic image with a first color developer composition toform the first color separation image, (II) forming a second colorseparation image over the first color separation image on the surface ofthe dielectric support by(iv) overall charging the surface of thedielectric support and first color separation image, (v) while thedielectric support and first color separation image are still charged,imagewise-exposing the photoconductive layer to actinic radiationthrough the substrate to form a second electrostatic image, and (vi)developing the second electrostatic image with a second color developercomposition to form the second color separation image, and (III)contacting the surface of the dielectric support having said colorseparation images thereon with a receiving element, and (IV)transferring the dielectric support and color separation images to thereceiving element to form a multicolor proof.
 2. A color proofing methodas described in claim 1 wherein said dielectric support comprises anovercoat releasably adhered to said photoconductive layer.
 3. A colorproofing method as described in claim 1 or 2 wherein, prior to step(III), steps (II)-iv through (II)-vi are repeated to form third andfourth color separation images, thereby forming a composite of fouroverlapping toner separation images on said dielectric support.
 4. Acolor proofing method as described in claim 3 wherein said first,second, third and fourth color separation images are black, cyan,magenta and yellow separation images, respectively.
 5. A color proofingmethod as described in claim 1 or 2 wherein said receiving elementcomprises a paper sheet.
 6. A color proofing method as described inclaim 1 or 2 wherein the developer composition employed in the formationof each separation image is a liquid electrographic developer comprisingan electrically insulating carrier liquid and toner colorant particlesdispersed in said liquid.
 7. A color proofing method as described inclaim 2 wherein said dielectric support is transparent.
 8. A colorproofing method as described in claim 2 wherein said photoconductivelayer contains a release agent to enhance the releasability of saiddielectric support from said photoconductive layer.
 9. A color proofingmethod as described in claim 8 wherein said release agent comprises ablock copolyester containing polysiloxane recurring units.
 10. A colorproofing method as described in claim 1 or 2 wherein saidphotoconductive layer is sensitive to actinic radiation below 400 nmonly and exhibits nonpersistent conductivity.
 11. A color proofingmethod as described in claim 10 wherein said photoconductive layercomprises an arylalkane leuco base photoconductor.